Subscriber loop transceivers used in baseband data transmission systems such as the integrated services digital network (ISDN) share a single transmission medium over which terminals may simultaneously transmit and receive data signals. In order to maintain simultaneous transmission and reception, it is necessary to remove interference due to signals being transmitted appearing at the input of the receiver and intersymbol disturbances so that the received signal may be detected. Decision feedback equalizers have been used to compensate for intersymbol interference in the received signal as a result of the frequency dependent attenuation characteristics of the transmission line and echo cancelers are used to remove the transmitted signal reflected back to the receiver input.
Generally, both decision feedback equalizers and echo cancelers have been of the adaptive transversal filter type. As is well known in the art, such transversal filters span a finite time period and can only compensate for an impulse response of finite duration. The finite duration response, however, is only an approximation to the actual response of the transmission channel for both the path of an echo and the path of the received signal whereby compensation is only effective for near term interference. The far term interference or tails of the echo path or received signal path impulse responses are beyond the span of the transversal filter and cannot be canceled. Consequently, there is a degradation of system performance.
Recursive type filters such as disclosed in U.S. Pat. Nos. 3,646,480 issued Feb. 29, 1972 to D. A. Spaulding and 3,798,560 Mar. 19, 1974 issued to M. G. Taylor describe arrangements using recursive filter structures for equalization which may also be used for echo cancellation. Such recursive filters, however, are more difficult to adapt than nonrecursive filters using the transversal structure since the gradient adaptation method for transversal type filters is unstable when applied to recursive type filters. Adaptation methods such as disclosed in U.S. Pat. No. 4,057,696 issued to R. D. Gitlin et al Nov. 8, 1977 overcome the instability problem but are complex and are best suited for voiceband echo cancelers or modems where the low sampling rate allows multiplexing of hardware.
At the high data rates used in ISDN transceivers, hardware requirements necessitate a simpler adaptation method. The article "Mixed Recursive Echo Canceller (MREC)" by C. Mogavero et al appearing in the Proceedings of the 1986 Global Communications Conference (GLOBECOM) at pages 44-48 and U.S. Pat. No. 4,755,984 issued to R. Ambrosio et al Jul. 5, 1988 describe a tail canceler structure in which a nonrecursive filter exhibits a finite impulse response of the "boxcar integrator" type. This canceler structure adapts its gain using a modified version of the gradient method. Such adaptation has a sufficient length to approximate cancellation but the impulse response is finite whereby the degree of cancellation is limited.
The article "Architecture for Fully Integrated Echo Canceller LSI Based on Digital Signal Processing" by Hiroshi Takatori et al appearing in Proceedings of 1987 International Communications Conference at pp. 601 through 605 discloses an arrangement in which the aforementioned "boxcar integrator" is replaced by an ideal integrator having a step function impulse response which is used in combination with a vector quantization technique to adapt the filter gain. As is well known, the response of a transmission line for the tail component is primarily dependent on the parameters of the transformer used to couple the transceiver to the line and is largely independent of the line characteristics. The infinite impulse response selected to simplify the hardware needed for implementation by Takatori et al, however, is a relatively poor approximation to this transmission line response resulting in incomplete cancellation. It is an object of the invention to provide improved echo and intersymbol interference cancellation with higher accuracy removal of the tail portion components thereof.